I have a 29' class C with the 6.0 chevy. We regularly tow a Toyota camry from Ohio to Eastern Shore of Maryland across I68, which is one mountain after another. We have no problem. I use the truck lane going up and maintain about 40-50mph, could probably do faster but see no reason to. Most of the climbs are 5-6%. Get about 9 mpg while towing the camry, about 10-11 mpg while not towing. Went out west this summer, had no problems there either. The climb out of Dayton WY on US14 I guess is about 7%, climbed it(not towing) at 25-35mph. Again, could have gone faster, but why? Gary